It is common for AC kWh meters to use current transformers to measure the current flowing into a load, from which the power may be derived by known means in conjunction with the measured voltage across the load.
Certain types of load containing half-wave rectifiers or triac-controlled devices may introduce a DC component to the current consumed, which may saturate the current transformer and reduce measurement accuracy. Some types of current transducer or processing circuitry may not pass DC levels. Thus some electricity consumers may obtain free power by this method. DC currents may cause saturation in distribution transformers, causing loss of efficiency, higher temperatures and reduced lifetime, as well as causing corrosion of water or gas pipes and ground rods, with consequent safety hazards, and maintenance costs.